Our recent experiments have demonstrated that the p53 gene is a suppressor of human tumor growth and that this gene is mutated in a large number of human cancers. These cancers include those of the brain, colon, lung, breast, ovary, bone and bladder; indeed, p53 is the most commonly mutated gene yet identified in human tumors, genetically altered in nearly half the total cancers that occur in the United States annually. Understanding the role of the p53 gene in human tumorigenesis therefore has become a critically important research goal. This proposal is designed to continue our investigations on p53 through a variety of approaches, including (i) clinical studies, to define the timing of p53 gene mutations during the course of human tumorigenesis, and the prevalence of inherited gene mutations in families predisposed to cancer development; (ii) physiologic studies, to examine the effects of wild-type and mutant p53 expression on the growth and cell cycle control of cancer cells containing endogenous p53 gene mutations; (iii) genetic studies, to discover genes that modulate p53 action in vivo, in both mammalian and lower eukaryotic environments; and (iv) biochemical studies, to identify DNA sequences and cellular proteins to which p53 binds in vitro and in vivo. Each of these approaches has the potential to illuminate specific aspects of p53 function. In combination, they should lead to new insights into the biology of a prototype tumor suppressor gene, with implications for diverse forms of human neoplasia.